Combinations.java

/*************************************************************************
 *  Compilation:  javac Combinations.java
 *  Execution:    java Combinations N k 
 *  
 *  Enumerates all subsets of size k on N elements.
 *  Uses some String library functions.
 *
 *  % java Combinations 5 3
 *  abc
 *  abd
 *  abe
 *  acd
 *  ace
 *  ade
 *  bcd
 *  bce
 *  bde
 *  cde
 *
 *************************************************************************/

public class Combinations {

    // print all subsets that take k of the remaining elements, with given prefix 
    public static void generate(String prefix, String elements, int k) {
        if (k == 0) {
            System.out.println(prefix);
            return;
        }
        for (int i = 0; i < elements.length(); i++)
            generate(prefix + elements.charAt(i), elements.substring(i + 1), k - 1);
    }  

    // read in N and k from command line, and print all subsets of size k from N elements
    public static void main(String[] args) {
       int N = Integer.parseInt(args[0]);
       int k = Integer.parseInt(args[1]);
       String elements = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
       generate("", elements.substring(0, N), k);
    }

}